Two endogenous neuropeptides (SCP_A and SCP_B) produce a cAMP-mediated stimulation of cardiac activity inAplysia

Summary

• 1. Two endogenous neuropeptides (SCP_A and SCP_B) have recently been isolated fromAplysia tissue and sequenced (Fig. 1).

• 2. In the present study, we show that the SCPs excite isolatedAplysia hearts with a threshold below 10⁻¹¹ M (Figs. 2, 4). The SCPs increase both the beat amplitude and heart rate in a manner similar to serotonin (Fig. 3), although the peptides are at least 10-fold more potent. The actions of the SCPs appear to be quite specific as several peptides with sequences moderately similar to the SCPs were completely inactive on the isolatedAplysia heart (Fig. 1).

• 3. The effects of serotonin on the heart appear to be mediated by an increase in cAMP levels primarily in the AV-valves (Mandelbaum et al. 1979). Long term incubation (5 min) with SCP_B produces moderate increases in cAMP levels in the auricle and ventricle but not in the AV-valve (Table 1). We therefore determined if the contractile activity of the AV-valve was enhanced by SCP_B.

• 4. SCP_B increased the rate and amplitude of the contractions of an isolated spontaneously beating AV-valve in a manner similar to serotonin except that the responses to SCP_B desensitized rapidly (within 2 min; Fig. 5).

• 5. cAMP determinations from AV-valves with shorter exposures to SCP_B revealed very large increases (∼ 100-fold) with short exposures (15 s) that rapidly decreased to near control levels with longer exposures (within 2 min; Fig. 6). Thus, the effects of SCP_B on cAMP levels and on the rate and amplitude of the AV-valve contractions showed similar kinetics. The effects of SCP_B on cAMP levels in the ventricle were also time-dependent but much less so than for the valve (Fig. 6).

• 6. These data support the proposal that excitation of theAplysia heart is mediated by increased cAMP levels. To further test this proposal, we exposed the isolated AV-valve to the adenylate cyclase activator, forskolin. 10⁻⁵ M forskolin produced an increase in the rate and amplitude of valve contractions similar to that produced by serotonin or SCP_B (Fig. 7). The response to forskolin did not desensitize.

• 7. Using a sensitive HPLC-bioassay procedure, neither SCP_A nor SCP_B were detected in the auricle, AV-valve, or ventricle. Furthermore, neither peptide could be detected in the blood using a procedure with a maximum sensitivity of 10⁻¹¹ M. These findings suggest either that the SCPs regulate the heart at blood concentrations below 10⁻¹¹ M, or that the heart is regulated by some other substance that is similar to the SCPs.